The telephone subscriber loop is the portion of a telecommunications circuit which connects the central office to the customer premise equipment (CPE). Often referred to as the local loop or "last mile", the subscriber loop typically includes the wire, poles, terminals, conduit, and other outside plant items. The subscriber loop wiring comprises a two-wire twisted-pair cable to provide full duplex communications between the subscriber and the central office.
The local loop is the most expensive, and perhaps the least technically effective, portion of the entire telecommunications system today. The subscriber loop is also the most susceptible to transmission impairment, which is largely due to the subscriber loop being exposed to the elements as it hangs on poles or is buried in underground conduit to reach the end user. Adverse weather conditions, defective cable insulation, wet cables, and damaged or severed buried cable are just a few of the circumstances that can lead to degradation or total breakdown of normal telephone communication.
One principal type of fault which can occur on the subscriber loop is a leakage fault. For example, the tip and ring lines comprising the twisted-pair in the subscriber loop could be coupled through some finite amount of impedance. Leakage between the ring and/or tip line and ground is also an undesirable condition. This condition may degrade the optimal voltage differential between the tip and ring lines, and can also introduce very undesirable noise onto the communications line. Also, it can provide an early indication of an impending line failure.
There have traditionally been three categories of telephone subscriber line testing. First, local test desks (LTD) have been used to access a cable pair through the switching system at the central office. The LTD is a manually operated system which accesses the cable pair through a circuit in the switching system that connects the LTD to the line without ringing the telephone. Such a test requires dedicated test circuitry and a test board operator at the central office.
Another type of telephone subscriber line testing is automatic test equipment that accesses local loops through the switching system, similar to that of the LTD tests. Such a test is the line insulation test (LIT), which is performed at the central office by stepping through each telephone line and applying a line test.
Automatic test gear requires a relay on each telephone line to provide test access to the outside line. The test gear is generally contained in a separate rack, and is fairly expensive. This cost can be justified because, in a central office, it is shared among perhaps thousands of lines.
A modern trend is to move the interface to the subscriber loop into a remote terminal closer to the customer. The remote terminal is then connected to the central office switch gear by a digital link, which might use copper, fiber optics, or even radio. As the remote terminals are moved closer and closer to the customer, they also contain fewer and fewer telephone lines, until ultimately it may reside inside a customer's house and serve only one or two telephone loops inside the house. This process causes the cost of the test gear to be spread over fewer and fewer lines. Unfortunately, existing subscriber line circuits have never contained more minimal test circuitry. One subscriber line interface circuit is the advanced subscriber line interface circuit (ASLIC) Am79212/Am79C202 commercially available by Advanced Micro Devices, the assignee of the instant application.
However, the aforementioned prior art does not provide for subscriber line diagnostic testing which utilizes the existing current drive circuitry provided to drive the telephone connection during normal operation. The prior art typically includes separate circuitry, and often a separate diagnostic rack, to perform diagnostic testing. The prior art requires additional, valuable semiconductor real estate. The present invention provides a solution to these and other shortcomings of the prior art, and further provides additional advantages over the prior art.